When three or more telephones are connected together in a conference connection with no special conferencing circuitry, a loss of signal strength is experienced compared with the signal strength in a typical two telephone connection. When the crosspoints of the switching matrix have no resistance, the loss is caused by the additional impedance load to which a transmitting telephone must supply signal power. When the crosspoints have a nonzero resistance, additional loss is caused by the power lost in transmission through the closed crosspoints. When the crosspoint resistance is a known, fixed value, loss can be eliminated by the use of a monobus circuit, which splits outgoing signals from incoming signals into current and voltage signals, respectively.
Such a monobus circuit is disclosed in the concurrently filed, copending patent application of D. J. Morgan and D. C. Smith Ser. No. 580,292, filed May 23, 1975 where the monobus interface circuit is interposed between each line and the switching matrix. The interface circuit is a current source arranged to provide the current equivalent of the voltage signal available from the transmission source. In the above-identified Morgan-Smith patent application, one bus resistor is supplied by the switching matrix and is shared in common by the monobus circuits serving the other connected transmission lines. In this manner the current signal generated by the port circuit current source is converted to a voltage signal by flowing through the common bus resistor and supplied via the switching matrix to each other connected port circuit.
When the switching matrix crosspoint resistance is negligible, the component of the bus voltage measured at the current source output which results from current generated from each line is equal and opposite in magnitude to the voltage generated in the line. In any given line these voltages cancel each other, thereby eliminating the feedback signal. In situations where it is desired to use crosspoints having variable resistance, the voltage returned from the bus resistor plus the crosspoint resistor does not exactly match the locally generated voltage and thus a feedback signal of unknown magnitude is applied to the telephone line, causing the return loss to be different than in the known or zero crosspoint resistance case, and the transmission loss to be modified due to the resultant feedback signal.
Thus, it is an object of our invention to provide a monobus hybrid port circuit for use with a variable resistance switching matrix where the crosspoint resistance does not affect the transmission signal levels.